Input system for electrical



March 17, 1936. F. H. DRAKE ET AL. Re- 19,892

INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Original Filed June 16, 1928 5 Sheets-Sheet 1 GIN March l 7, l 936.

F. H. DRAKE ET Al..

INPUT SYSTEM FOR ELCTRICAL AMPLIFIERS Original Filed June 16. 1928 3 Sheets-Sheet 2 atroz e134 March 17, 1936. v F, H, BRAKE ET AL Re. 19,892

' INPUT SYSTEM FOR ELECTRICAL AMPLIFTERS original Filed June 16, 192B s sheets-sheet 3 200 50a 400 @oIs-a attorney Ressued Mar. 17, 1936 UNITED STATES PATENT OFFICE INPUT SYSTEM FOR ELECTRICAL AIVEPLIFIERS Original No. 1,843,018,

Serial No. 285,886, June 16, 1928.

dated January 26, 1932, Application for reissue January 26, 1934, Serial No. Il08,3'15

4 Claims.

VThis invention relates to electrical amplier circuits, and more especially to an input system for such amplier circuits. It is particularly applicable to an input system for a radio frequency 5 amplifier of a type suitable for use in radio receiving sets, and the invention will hereafter be described in' connection with such an amplifier, in which connection it has many advantages.

An object of the invention is to provide such an input system constituting a desirable means for coupling an antenna or other collecting device to a radio frequency amplifier. A further object is to provide such an input system including a volume control. Other objects of the invention will appear from the following description.

Figure 1 is a diagram of one form of input system embodying the invention and showing the antenna and ground connections. Figure 2 shows a similar input system including a volume control. Figure 3 shows a multi-stage radio frequency amplifier including an input system similar to that of Figure 2. Figure 4 shows one construction of a trimmer for use in an input system according to the invention. Figure 5 is a graph showing curves of the wavelength characteristic of certain input systems.

Referring now to Figure 1, the input system comprises a tuned circuit including in series a xed condenser Cm, an inductance L of adjustable value (i. e. a trimmable" inductance or trimmer) and a variable tuning condenser Co connected across said xed condenser and trimmable inductance. The terminals I and 2 of the tuning condenser C0 may suitably constitute the input terminals of a radio frequency amplifier, for example the grid and cathode of the rst tube. 'I'he antenna may be connected or coupled to a point between Le and Cm, the other side of Cm being grounded.

Although the invention is not limited to any particular Values for the several circuit elements, the following approximate values (here given as an example only) have been successfully em- 45 ployed:

Cm 1000m# Co (max) =400llf Lo =170240 m. h. (trimming range) In other cases a valve of Cm=2000aaf has (Cl. Z50-20) It will be observed that the total tuning capacity of the input system, with the antenna removed, isz- The antenna capacity is effectively in parallel with Cm and acts to increase the effective value of Cm in the above equation.

If Cm is large compared with C0, as is the case when the circuit is tuned to short wavelengths, the term C.. 1+Cm (i. e. is negligible in comparison with 1) for example at short wavelengths. At the same time, at short wavelengths the Voltage, e, between antenna and ground, is applied to the tuned circuit Lo-Co-Cm by means of the coupling between that circuit and the antenna through the capacity Cm. Cm being fixed, this coupling decreases at short wavelengths, and increases at long wavelengths. As a result of this arrangement, the wavelength characteristic of the input system is of more desirable form than has been hitherto obtainable, and by proper proportioning of the constants of this circuit, the wavelength characteristic may be made uniform or substantially constant over the wavelength range for which the system is designed.

Figure shows certain wavelength characteristic curves, obtained by plotting Eo/e (i. e. the ratio of the voltage e induced in the antenna by the signal, to the voltage Eo across the tuning against wavelength for an input system according to the present invention, as obtained with antenna Ao, which was a single-room indoor antenna approximately 25 feet in length and 100ML! capacity. Curve B' is a typical characteristic curve, obtained with the same antenna A0, for an input system according to the prior artin which the antenna is coupled td a tuned input circuit either through a. small series capacity, -or through a small coil coupled to a coil of `the tuned input circuit, either prior art arrangement resulting in substantially the same characteristic curve. Curves C and C are similar curves for the same input systems but were obtained with antenna A1., which was a short outdoor antenna of approximately 250,144 capacity.

In the -case of input circuits of `the type involving inductive coupling and fixed inductances tuned to resonance at different wavelengths by variable condensers, as has heretofore been the usual arrangement, the wavelength characteristic curve ordinarily has the lshape of curves B and C of Figure 5. This shape of characteristic curve is undesirable, especially in the case of input systems for use with radio frequency amplifiers, for a number of reasons. For example, such amplifiers are much more apt to be unstable at short wavelengths than -at long wavelengths, due to the increased capacity and magnetic couplings in the amplifier at short wavelengths. To avoid instability, excessive sensitivity, or tendency to oscillate, it is frequently desirable to impart to such an amplifier, if possible, an amplification characteristic curve which is substantially constant over a range of wavelengths, -or which at least has a minimized rise at short wavelengths. Whether or not such a characteristic is imparted to the amplifier, Vit is highly desirable that the input voltage impressed upon the input terminals of the radio frequency :amplifier should not -be very greatly higher at short wavelengths than at long wavelengths, and it is preferable that the voltage should be substantially constant :over 'the wavelength range of the amplifier, since otherwise the undesirable sloping characteristic of the radio frequency amplier, already higher at short wavelengths than at long wavelengths, will have superimposed upon it another undesirable sloping characteristic of the same type, making an over-all amplification which is so much higher at short than at long wavelengths as to produce oscillation, instability, excessive sensitivity and other undesirable effects. For the purpose of avoiding this condition, it is desirable therefore to employ an input system having a wavelength characteristic curve resembling the curves shown at B and C of Figure 5, which curves were obtained with an input system according to the present invention designed for .use in a broadcast radio yreceiving set, and which are substantially constant over the broadcast range of wavelengths from .20D-550 meters. Such wavelength lcharacteristics are possessed by .input systems according `to the present invention, and constitute an important advantag thereof.

Figure 2 shows the invention applied to an input system including a volume control arrangement as an integral part thereof. 'Ihis is provided by the resistance R connected across the fixed capacity Cm, and provided with a variable tap connected to the antenna. When this antenna tap is at the top of the resistance R, the antenna has maximum coupling to the input circuit through Cm so that the full incoming signal is impressed upon a succeeding amplifier, and as the tap is moved down along R the antenna is connected more and more nearly directly to ground, so that less and less of the received signal is impressed upon an amplifier with which the linput system is associated. At the bottom position of the tap, the antenna is grounded, and there is substantially no coupling to the input system.

In one such arrangement, which has been found Vto give Ian excellent volume control effect, the resistance R had a value of 50,000 ohms when the .other constants of the input system were as :given Ain ythle above example in connection with Figure 1.

It to be noted that the volume control arrangement dascribed above is -not .of general applicability to tuned input systems of the types hitherto employed, but can only be successfully employed, so far as is .now known, with an input system according to the present invention, since the antenna. lcapacity is verysmall as compared to the value of the fixed capacity Cm. The adjustment of the resistance R 'therefor exercises substantially no detuning effect.

Figure 3 shows the combination of an input system and volume control arrangement according to the present invention with -a tuned radio frequency amplifier of the type described and claimed in United States Patent No. 1,672,811, issued June 5, 1928. In this arrangement T1, 'T2 and T3 are three electron or vacuum tubes in three radio frequency amplifier stages working into a detector stage including .tube T4. An input system La-CU-Cm-R according to the present invention is arranged between grid G and cathode F (which may be grounded as shown) of the tube T1 inthe first radio frequency stage. Each tube has .a plate coil L1 which is coupled or connected so as to have effectively negative mutual inductance with thesecondaryrcoil L2. In series with La is .connected a tertiary coil L3, having negligible coupling with the coils L1 and L2, for which purpose .it ymay be arranged at right angles thereto, and a tuning condenser C2. A balancing capacity C3 -is connected between the control grid G and the junction vof coils L2 and L3, while the cathode Fis connected (ordinarily through ground) tothe junction of the -coil La and the tuning condenser C2. Common A and B batteries, or equivalent socket-power devices may -be employed for the several stages, being suitably by-passed if desired; or .alternating current filaments or cathodes of the separate heater -types may be employed.; and the grids YG may,if desired, be biased lby means of a suitably arranged C batteryor by a biasing resistance in the filament circuit. 'I'he detector stage, including tube T4, may be arranged in any suitable Way, being `provided for instance, with grid condenser Cg, grid-resistance Rg, and radio frequency by-pass condenser C', the detector output terminals being suitably connected to an audio frequency amplifier and loudspeaker.

Such an arrangement constitutes a :balanced multi-stage amplifier, as described and claimed in above mentioned United States Patent No.

1,672,811, and is adapted for operation by a single control, all of the condensers Co, C2, Ca, C2, being units of a gang. 'I'hey are then operated together, and it is desirable that for any given motion of the single control which operates these condensers, each condenser shall have the same increment or decrement of capacity as all the other condensers. When such is the case, all of the circuits Lz-La-Cz tune together and resonate to the same frequency. In order to make the circuit LU-Co-Cm-R tune and resonate with the other tuned circuits, i. e. so that the circuits tuned by condensers Co and C2 track or remain in alignment, the inductance Lo may be trimmed or adjusted, when necessary, by a suitable trimming adjustment, thereby obtaining exact resonance of the input system. 'Ihis trimming adjustment may be employed either to take care of different antennas orother collecting devices employed in connection with the input system (as, for example, when a change is made from antenna Au to antenna A1 of Figure 5, or when a single model of broadcast radio receiving set is sold to many different purchasers each erecting his own collecting device) or, in tuning the amplifier to dierent wavelengths on a single antenna, to keep the input system in resonance at the same wavelength as the other tuned circuits. Thus the trimming of Ls takes care of departures in the value of C@ from substantial equality With the value of Co for values of C0 where Co/Cm is no longer negligible compared with unity.

In such an arrangement as that of Figure 3, when the radio frequency amplifier is embodied in a radio receiving set, the set normally possesses three panel controls; namely a main tuning control which operates the condensers Co, Cz, C2, C2 as a gang, a trimmer which varies the inductance of L0, and a volume control which operates the tap on R. Such a control system for a radio receiving set has been found to be unusually satisfactory, and is now widely used commercially.

The particular multi-stage arrangement of Figure 3 is here shown by way of example only of the combination of an input system with a singlecontrol multi-stage tuned radio frequency amplifier, according to the present invention, and the invention is, of course, not limited thereto.

One suitable arrangement and construction for such a trimmer is shown in Figure 4, in which the coil L0 is wound on a cylindrical form and mounted on the chassis or panel of a radio frequency amplifier. The inductance of the coil La is adjusted or trimmed by means of the conducting sleeve 3, suitably composed of copper or the like. This sleeve is moved back and forth over the coil Le, so as to surround more or less of it, by means of the push rod 4 moving in guide 5, and operated by the lever or crank 6 and control knob 1. In one such construction which has been successfully employed, the coil Lo had a diameter of approximately one inch, being composed of about 135 turns of #34 enamel wire, and the movable shield 3 had a diameter of about 11/2". This was found to give a decrease in the inductance of coil L0 of about 30% of its maximum value when the shield 3 was moved so as to substantially cover the coil.

In addition to the advantages already described as resulting from an input system according to the present invention, the following advantages may be mentionedz-The amount of trimming which is required is very small compared with that which is frequently necessary with other arrangements, and this results in extreme ease in tuning to different wavelengths, and in picking up different broadcast stations with a radio receiving set embodying the arrangement. When the volume control is changed the resultant detuning of the system is very small. It is an easy matter to accommodate the system to different types of antennas or other collecting devices. At short wavelengths the coupling of the input system to the antenna is weak, which is desirable, as pointed out above, since excessive ampliications are not required at short wavelengths. Moreover, the selectivity of the system is high at short wavelengths, Where an ordinary radio frequency amplifier is ordinarily relatively non-selective. In the case of a trimmer arrangement such as that shown in Figure 4, the position of the conducting sleeve at short wavelengths is over the coil, and the sleeve thus operates as a shield for the coil at those frequencies when shielding is particularly required.

Other advantages of an input system according to the present invention will have appeared from the foregoing description; and the invention is not limited to the particular arrangements or constants herein shown or described, but extends to many variations and modifications thereof, and is limited only by the teachings of the foregoing specification, by the prior art, and by the following claims.

We claim:

1. A tunable input system for an audion amplifier comprising the combination with a tuned circuit including in series a fixed condenser, an inductance and a variable condenser, of a resistance shunted across said fixed condenser, and a variable tap on said resistance, said tap and the junction of said fixed and variable condensers serving as terminals across which is impressed the incoming alternating current voltage that is to be amplified, and terminals of said variable condenser serving as output terminals across which said amplifier may be connected.

2. An input system for a radio frequency amplifier, comprising a collecting device; a tuned circuit including in series a fixed condenser, an inductance of adjustable value, and a variable condenser connected across both said inductance and said fixed condenser; a resistance connected across said fixed condenser; a connection from said collecting device to said resistance including a variable tap, said connection being arranged to constitute a volume control for an amplifier with which said input system is adapted to be associated; a connection from said fixed condenser to ground; and means for applying the voltage across said variable condenser to the input terminals of a radio frequency amplifier.

3. In radio frequency apparatus, a source of high frequency energy and a load circuit, means for coupling the source to the load circuit comprising a high frequency circuit having an input terminal, an output terminal and a common terminal, a fixed capacity connected directly between the input and common terminals, an impedance device connected across the xed capacity, means for connecting the source of high frequency energy between the common terminal and a point of the impedance device, a variable condenser of smaller capacity than the fixed capacity connected between the output and common terminals, and an inductance device connected between the input and output terminals, said fixed capacity, variable capacity and inductance device forming a closed circuit which is tunable over a frequency range, the elements of the closed circuit being proportioned to provide lmon terminal mdtheouhputznnhmLam capacity connected directly between 'the input terminal and the common an impedance device connected a'crn itine md capacity, the source of high frequency being connected between the 'outlmt temnnl md l. pmt

IMU

1 Ydevice icing minable, .a variable condenrcf maiierespitytm the fixed capacity ed vbetween the `output and common te und 'an vi:lruzh1ctance device connected between the input und output terminals, said indevice, variable condenser and xed clpacity farming m. closed circuit tunaible over a frequency range, the elements of said closed circu'rt being proportioned to provide uniformly 10 high 1 and sensitivity throughout said fneqnemy time, a @variable condenser device in the alecm-ic mplmg `circuit `connecting said tubes m'cascade and l, :single :means for varying the cmcity ful the two variable condensers in ll Umm.

IBEIIBICK VIH. DRAKE.

D. LOUGHLIN.

f device, the :connection kto the f 

